The rational development of new antineoplastic agents directed against tubulin, a protein critical for cell division, requires greater understanding of the interaction between the polypeptide subunits of tubulin, its two tightly bound guanine nucleotides, and microtubule-associated proteins. Copolymerization of tubulin.GDP and tubulin.GTP was observed, and reaction conditions which favored incorporation of tubulin.GDP into microtubules were defined. Instability of microtubules to phosphofructokinase and fructose-6-phosphate after exhaustion of GTP in the reaction mixture could not be attributed to breakdown of nonexchangeable GTP bound to tubulin. Continued progress was made in the separation of Alpha-tubulin and Beta-tubulin on a preparative scale; and in the purification of microtubule-associated proteins which cause the formation of microtubule bundles and nucleotide interconversions.